Integrated circuits include many discrete semiconductor devices. A multi-level network of metal within dielectric overlies and connects these discrete devices. For many years, the metal used was generally aluminum or an aluminum alloy. More recently, copper has been used in place of aluminum because copper's higher conductivity improves circuit performance.
Two major obstacles had to be overcome before copper could be used in integrated circuits. First, copper is difficult to etch in order to form wiring patterns. Second, copper diffuses rapidly. Copper can diffuse into silicon where it can cause junction failure. Copper can also diffuse through dielectric layers, degrading them and eventually traveling though them into device regions.
The problem of forming copper wiring patterns has been successfully overcome using damascene processes. In a damascene process, openings that form an image of an interconnection pattern are patterned in a dielectric layer. Copper is deposited or grown within these openings. Polishing is used to coplanarize the dielectric layer and the copper. This leaves a copper interconnection pattern inlaid within the dielectric layer. In a single damascene process, the dielectric is patterned through. Inter-level contacts are formed with one deposition and polishing step, and wiring with another deposition and polishing step. In a dual damascene process, the dielectric is patterned with both trenches and vias, whereby both inter-level contacts and wiring can be formed with a single metal deposition and polishing step. Regardless of which type of process is used, multiple layers are formed to create complex interconnection patterns.
The problem of copper diffusion into silicon and dielectric layers has been overcome using diffusion barriers. Layers of diffusion barrier material are provided between copper and adjacent dielectric or silicon. A variety of barrier materials have been reported. Conductive barrier materials include compounds of transition metals such as tantalum nitride, titanium nitride, and tungsten nitride as well as various transition metals themselves. Dielectric barrier materials include silicon nitride, silicon oxynitride, and PSG (a phophosilicate glass).
While the forgoing solutions have been implemented and copper has now been used for some time in integrated circuits, there is a continuing need to improve integrated circuit performance. For several years, efforts to improve integrated circuit performance have focused on replacing conventional dielectric materials, generally silicon dioxide, with low dielectric constant (low-k) materials. These materials provide a lower capacitance than silicon dioxide and consequently, increase circuit speed by decreasing the corresponding RC delay. Low-k barrier materials, such as SiC, are generally used with low-k dielectrics to achieve the goal of lowering overall capacitance.
Unfortunately, difficulties have arisen when patterning low-k dielectric layers. Low-k dielectric layers are patterned using lithography. Lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, the substrate or dielectric or other film to be patterned is coated uniformly with a radiation-sensitive film, the resist. The film is selectively exposed with radiation (such as visible light, ultraviolet light, x-rays, or an electron beam) through an intervening master template, the mask or reticle, forming a particular pattern. Exposed areas of the coating become either more or less soluble than the unexposed areas, depending on the type of coating, in a particular solvent developer. The more soluble areas are removed with the developer in a developing step. The less soluble areas remain on the substrate forming a patterned coating. The pattern of the coating corresponds to the image, or negative image, of the reticle. The patterned coating is used in further processing of the substrate, dielectric or other film.
One type of photoresist is a chemically amplified deep UV photoresist. A deep UV photoresist often is employed because resolution in lithography systems is primarily limited by diffraction of radiation passing through the reticle. Employing the small wavelengths of deep UV light reduces diffraction, however, it is difficult to produce deep UV light at high intensity. To compensate, a chemically amplified photoresist is used. In a chemically amplified photoresist, the radiation generates a catalyst, typically an acid, which catalyzes a solubility-changing reaction that occurs during a post-bake operation following selective exposure of the photoresist to actinic radiation. Sometimes contaminants can occur which may impact negatively the pattern of the resist, which then may be transferred to an underlying material during subsequent patterning.
Attempts have been made to remove contaminants from low-k dielectrics using conventional techniques, such as washing with solvents. Unfortunately, conventional techniques have not proven effective in preventing contamination of chemically amplified photoresist formed over low-k dielectrics. There remains an unsatisfied need for a method of dealing with contamination when low-k dielectrics are employed.